Get the Whitepaper called The Shellshock Attack. It covers all of the required topics for understanding the Shellshock vulnerability. The proof of concept will help visualize and perform the attack in a virtual scenario to understand the attack vector and the process of exploitation.
You can download it from the following link: https://packetstormsecurity.com/files/download/156488/the-shellshock-attack.pdf
Source: https://packetstormsecurity.com/files/156488/The-ShellShock-Attack.html